This application claims the priority benefit of Taiwan application serial no. 89115833, filed Aug. 7, 2000.
1. Field of Invention
The present invention relates to a method for improving the monitored range of a flow sensor. More particularly, the present invention relates to a flow sensing system capable of monitoring a more narrow range so that manufacturing stations can have a more stable supply of fluid.
2. Description of Related Art
Aside from some very special gases or solvents, most stations for manufacturing semiconductors require supply chilling water or gaseous nitrogen. Consequently, fluid control is important for carrying out special types of reactions as well as operating most processing machines.
The most convenient and direct means of controlling fluid is to measure its flow rate. Because different types of gases or liquids are used in different semiconductor processing stations, many types of fluid flow monitoring and controlling systems are manufactured in the semiconductor industry.
FIGS. 1A-1D is a sketch of a conventional flow sensing system. As shown in FIG. 1A, the flow sensor includes a transparent tube 18 having a marking at a definite height. The marking 16 is located at the target flow level. A group of sensors 10a and 10b are attached to the exterior sidewall of the transparent tube 18. The sensors 10a and 10b operate as a light-block sensing system.
Refer to FIG. 1A, for example, a beam of light 14 emitted from the light sensor 10a on one side of the transparent tube aims at the sensor 10b on the opposite side of the tube 18. The sensors 10a and 10b are positioned at a level of about 80% of the height of the target flow rate 16. The sensing system also includes a floating buoy 12 inside the transparent tube 18. The position of this floating buoy inside the transparent tube 18 indicates the current flow rate.
Before a particular manufacturing process is carried out, as shown in FIG. 1B, fluid must be fed to the operating station via the transparent tube 18. Fluid flow rate is indicated by the height level of the floating buoy 12 inside the transparent tube 18. In other words, the higher the floating buoy 12, the greater the fluid flow rate will be. When the floating buoy 12 reaches a level at about 80% of the target value 16, light 14 sent from the light source 10a will be blocked by the buoy 12.
Refer to FIG. 1C, As an increase in flow rate causes the floating buoy 12 to rise further such that a top side of the buoy 12 will reach the level of the target value 16. In the stage, the flow rate reaches the target value 16. Refer to FIG. 1D, as an increase in flow rate causes the floating buoy 12 to rise further such that the buoy 12 surpasses a level of about 130% of the target value 16, light from the light source 10a is still blocked by a button portion of the buoy 12.
When emitted light from light source 10a is blocked, this indicates the current flow rate is within the correct range. Conversely, any unimpeded light from light source 10a to the light sensor 10b indicates an out of range condition. That means, either too little fluid (as shown in FIG. 1B) or too much fluid (as shown in FIG. 1D) is passed to the processing station. When an out of range condition occurs, an alarm next to the machine is triggered warning an operator to pay attention to the situation before something serious happens to the processing station or the items inside the station.
FIG. 2 is a timing chart showing the fluid flow condition in a conventional flow sensing system. At time T0, the floating buoy rises gradually indicating the flow rate is also increasing. When the floating buoy reaches 80% of the target value, a waiting period is set aside for the fluid flow to stabilize. At time T1, the floating buoy reaches the target value and hence the sensing system starts monitoring the fluid flow. The monitoring continues until time T2. In other words, the sensing system must detect the presence of the floating buoy within the period T1xcx9cT2, which indicates that the fluid flows within the permissible range. On the other hand, if the sensing system does not detect the floating buoy within the period T1xcx9cT2, fluid flow is either too slow or too fast. In that case, an alarm is triggered to warn the personnel before irreversible damages are done.
At a pre-defined period after time T2, fluid flow will gradually fall to zero. If the floating buoy is at a position under 80% of target value, the sensing system detects no buoy. Conversely, if the floating buoy can still be detected by the sensing system, the fluid is still flowing at a considerable rate. Since this is an indication of some malfunction, an alarm is also triggered to inform the operator about this situation.
In general, a conventional flow sensor is limited by rocking stability of the floating buoy. Hence, the sensor is normally set to action at about 80% of the target value to prevent accidental tripping of a machine due to the rocking motion. However, due to considerations such as the size and length of a buoy, the actual controlling range is between 80% to 130% or even higher. That means, it is difficult to tailor a conventional fluid monitoring system to achieve a more narrow monitoring range and higher stability.
Accordingly, one object of the present invention is to provide a flow sensing system capable of monitoring a more narrow range (as little as 85% to 115%) so that stability demanded by the processing operation is improved.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of improving the monitored range of a flow sensing system. First, a target flow value, an upper limit and a lower limit are set. The flow rate is sensed in a first timing session. The detected flow rate is considered normal if the value reaches the lower limit within the first timing session. The flow rate is sensed in a second timing session. The detected flow rate is considered normal if the value reaches a level higher than the lower limit within the second time period. The flow rate is sensed in a third timing session. The detected flow rate is considered normal if the value drops to zero before the end of the third timing session. Throughout the whole process, the flow rate must not exceed the upper limit.
This invention also provides a flow sensing system capable of improving the monitored range. The system includes a transparent tube, a lower edge sense device, an upper edge sense device and a floating buoy. The transparent tube serves as a fluid flow monitor. A target value, an upper limit and a lower limit are marked on the transparent tube at suitable locations. The lower edge sense device is attached to the external sidewall of the transparent tube coinciding with the lower limit mark. The upper edge sense device is attached to the external sidewall of the transparent tube coinciding with the upper limit mark. The floating buoy is placed inside the transparent tube subjected to the pressure provided by a moving fluid.
The floating buoy indicates fluid flow rate. Under normal operating conditions, the lower edge sense device should be able to detect the floating buoy only once in a first timing session; the lower edge sense device should not detect the floating buoy at all in a second timing session; and the lower edge sense device should be able to detect the floating buoy only once in a third timing session. Meanwhile, under normal operating conditions, the upper edge sense device should not detect the floating buoy at all in any of the timing sessions.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.